Add Rollback Selected button to staging page

Rollback restores selected unstaged files to their HEAD state
using git checkout. The button shares the form with Stage Selected,
distinguished by the submit button's name/value pair.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/main/resources/templates/changes.html

@@ -19,7 +19,8 @@
 </tr>
 </table>
 <br>
-<input type="submit" value="Stage Selected">
+<input type="submit" name="action" value="Stage Selected">
+<input type="submit" name="action" value="Rollback Selected">
 </form>
 <p th:if="${#lists.isEmpty(modifiedFiles)}"><i>No modified files.</i></p>